Add support for sparse datasets

[charlesons]

• Adjust the data pivot logic that extracts series from tidy data so it retains the X values for each series and passes them through to the chart definition logic
• Adapt the stacked bar chart totals logic to support sparse datasets
• Add check for empty chart data in chartDefinition.ts after switching from map to chart types
• Refactoring and commenting the data loading logic for improved code readability

Closes #139

[rossbowen]

Walked through sparse and non-sparse instances and demonstrated the updates in use when dropped into Volto. Noticed one small NaN appearing when performing some logging which warrants investigation though things looked to be working correctly during the demo.